Neutronic reactor system



oct. 15,1957 F. DANIELS j 2,809,951V

NEUTRNIC REACTOR SYSTEM 3 Sheets-Sheet 1 Filed 06f.. 11, 1945 Oct. 15, 1957 F. DANI-ELS 2,809,931

Nau'momc REAcToR SYSTEM Filed oct. 11, 194s s sheets-sheet 2 Filed Oct 11 1945 v 3 Sheets-Sheet 3 F DANIELS NEUTRONIC REACTOR SYSTEM es r www! VZw z nited States liEU'IROlSlICA REACT'OR SYSTEM` Application October 11, 1945, Serial No. 621,845

2 Claims, (Ch.Y 204.-1932) The present: invention relates to atomic power plants and more particularly to aneutronic reactorfrom which heat can be removed to produce power in useful form.

In neutronic reactors aV neutron iissionable isotope such as U233, U235 or 942.39 or mixtures thereof is subjected to nuclear fission byr absorption` of slow neutrons, and a self-sustaining chainy reaction is established by the neutrons evolved:` by the fission. In general such reactors may comprisebodies 01E-compositions containing fissionable material, such as for example, natural uranium containing .7 percentof H235 disposed in a regular geometrical pattern in: at neutron. slowing. material or moderator. Graphite and' beryllium: are typical moderators suitable for such use. Heat is evolved during the ssionreaction and is customarily removedl by.- passage ofV a coolant through the reactor usually irrheat` exchange relationship with the uranium. ln such reactors the-transuranic element: 9.4 (plutonium) is. formed as a by-productl of the reaction.. Specific details ofl the theory and essential characteristics of such reactors areset forth inicopending applicatiorrof Enrico Fermi andzLeo Szilard, Serial No. 568,904, tiled: December 19, 19.44; now U. S; Patent 2,708,656.

Gases (aswell as liquids) have-been utilized as a coolant; tovcarry away thelheattof: reaction.. When gases have been used they,V were. forcedd through definite channels in the moderator in which the uranium ispositioned. The arrangement of-L such, channels was. determined. by the uranium body.- geometry and. for heaty removal at` high.

power outputs hasrequired the cooling gasy tobe applied athigh pressures ingorder to.force it; through the channels in suliiciently largeV quantitiestofobtain arequired temperature equilibrium. This type of gas cooled reactor; has been disclosedand claimedrin,- details in the. Fermi and Leverett application.SerialNumber.-578,278, tiled February 16, 1945, and the; present invention.v is an improvement thereon.,

The present invention is` primarilyconcerned; with'. the heating of gases by a novel type. Offneutronic reactor in which the. moderator and theuranium are inV separate unconformable units in the form- -of chunks; pebbles, approximate spheres, etc. for-` example mixedi together in random adjacent relationship but with a, definite overall averagervolume ratio., The reactor thus constructedhas desirably large and communicating voidspaces providing multiple, irregular and communicating channels extending throughoutY the entirereactorV in all directions. A cooling gas canA then be passedgthroughy the reactor with an exceptionally low pressuredrop-therein, and` in large volume. When the reactor is. operated at: temperatures from. above 100 C. to the-maximum of 2000 C., the exit gases can then beused togeneratefvapor under pressure, such as steam, inboilers of; substantially standard design or the heated gases can beused directly in gas turbines. In other` words such., at pebble b edircactor, as it may, becalled, canbe used, in place; of the iire. bed;V in boilers, with. only minor;designimodilication of standard boiler. design.

atent It is therefore an object*` of the present invention to provide a novel means and method of eti'ciently supplying hot gases useful in producing power, using the heating effect of a neutronic reaction inplace of the ordinarily used fuel combustion.

Another object of this invention is the provision of. a novel neutronic reactor ideally suited toY heat gases passing therethrough to-hightemperatures.

A still further object ofi this invention is to provide a novel neutronically reactive composition.

Other objects and advantages of they present invention will be more readily understoodv by'rreference tothe following description taken with thei attachedV drawings, wherein:

Fig. i is a diagrammatic sectional view partly in elevation of a pebble react-orpositioned toy supply hot'gases to a boiler;

Fig. 2 is a diagrammaticy View partly in section and partly in elevation of'l a closedf gas circulating system for the device shown inV Fig. 1; and

Fig. 3 is a diagrammatic View partly in section and partly in elevation of 'anl airv circulating system for the device shownfin Fig. l'. Y y

Thev structurev ofthe System will bedescribed' beforel taking-up the nuclear. physics ofthe system;

Referring iirst toFig; lf, a concretefoundation1I is provided resting on the earth 2'. supporting a: plurality of water; cooled` grates, 4; Grates. 4:- extend acrossa coned gas. space E. terminating below. in av discharge pipe-6`ll extending into` a discharge-.tunneh '7' positioned well belowl ground level. Y Around-1 grates 4;: is erectedi acylindrical wall retle-cton 9-of neutron; reflecting materialfin the-form of bricks or blocks.` Flhe. reflectori 9 isxsurrounded bya= cylindrical concrete side' wall 1:3., tive. toten feetthick, for example, erected onithe foundationi 1 which' isspaced from; thereector. 9 providing a-.cylindricalg absorber spacci 14. Absorber material may;be;dischargedf-intoatunnel 7r through an absorber outletpipe: 115i andvalve 16:. At cylindrical reactor space` 1-.7 isthus formed above;v the grates- 4 in whicha'novelneutronically-reactive composi- -tionis to be placed, as later described.

Above reactor space 17, converging concrete flue walls- 119are. provided, lined .with specialre brickZi ojfpressed beryllium, oxide on magnesium oxide; ofsutlicientthickess` to. protectthe surroundingy concrete. The" outleti 21E of the flue enters a boiler space 22finwhich watertubes 2.4.. are positioned connected in co11ventionalrnannerr to ani expansion chamber,` 25... The iiuef'gases pass over the tubes 24- several times, incustomary fashionV to emerge intoa gas outletpassage 2 6. "Ehe-water boiler described` is intended to. be, conventional, as. any. mode-rn; boiler adapted to handle. hot. gases. of the;temperatu'res'provided by thereactor. willbe satisfactory, andsthe example;shown.V is,v illustrative only; The entire boiler is covered by. a: concrete. radiation. shield 27. extended .fromtheiiue-conicretewhichis .lined withpressed.magnesiurrr oxide. Feedinlet 29, steamy outlet. 39. and; blow-olf pipe. 31 extend through radiationshield 27.- The passagedmay, beflined with pressed magnesiumuoxideif required1toprotect1the: concrete.

Beneath. grates 4.; a. gas inlet, 32 is, provided, entering5 coned gas spaceI 5.. Thus a.. clear ga.s.circ11lation path;is provided. through. grates.. 4, reactor.. space, 17,. the bollen spa-ce 22 and gas outlet passage 26.

The reactor, space 17. is,-to.be.lledv.with a neutronically active, composition consistingj primarilyVi of unconformable lumps on units. 34 oi a-.neutron moderator. suchasv graphite, or. sintered=j beryllium. oxide, and lumpson units: 35 of. a material containing a.- issionabler isotope; such asiuranium; metal, uranium: carbideor. sinteredg-. uraniunr oxide. A11 of thelatter units:35. containable isotope. Y

Patented Oct. 15, 19.57

H2355, a .fission- These discrete units 34 and 35 are preferably of approximately the same size and approximate shape, and are shaped so that they do not fit closely together. For example, rough spheres l to 3 inches in diameter are satisfactory and are loaded in random adjacent mix through loading opening 37 above the reactor space 17 to till the reactor space 17. With this arrangement the reactor space 17 will contain approximately 50 to 80 percent of solid material and 50 to 20 percent void spaces. The overall volume ratio however, of moderator to uranium is predetermined. Loading opening 37 is closed with a concrete and lire brick plug 38.

Control of the reaction is provided by use, for example, of a control rod 39 of a material having high neutron absorption such as cadmium, or boron, water cooled, which slides in a horizontal refractory sheath 40 of graphite or BeO. The control rod 39 is operated by rack and pinion 41, to insert more or less of the absorbel` into the reactor as desired. The neutron density is monitored, for example, by an ionization chamber 42. A last resort safety mechanism is provided by water jets 43 directed to the top of the reactor.

As it may be desirable to discharge some of the reactive units 34 and 35 from time to time, grates 4- can be operated in conventional manner to drop a quantity of units into coned space from which they roll into the discharge pipe 6. By the use of discharge Valve 44, which is preferably remotely controlled, the units are deposited in shielded cars 45 operating on tracks 46 in tunnel 7. The units may then be transported, if desired, to a chemical plant where the uranium is purified by removal of the radioactive fission products developed therein as the result of nuclear fission. Any element 94 produced by neutron absorption in the U238 content of the uranium may also be removed chemically from the discharged units when desired. For each amount of material discharged from the bottom of the reactor, fresh material can be added to the top of the reactor through loading opening 37, thereby maintaining the proper amount of reactive composition in the reactor at all times to insure the maintenance of the chain reaction.

The gas used as a heat transfer medium between reactor and boiler can be circulated through the reactor in a number of ways, two of which are shown in Figs. 2 and 3.

Referring first to Fig. 2, which diagrammatically illustrates a preferred type of closed circulating system using helium, the gas outlet 26 from the boiler is continued to a precipitating chamber 50 in which are located water cooled negatively charged electrode plates 51, in between which is a positively charged electrode 52. There may be a plurality of such electrodes 51 and 52 to form an electrostatic precipitator 53 of conventional design. Radioactive fission fragments entering the gas stream will be deposited on the water cooled plates 51 and are thus prevented from being recirculated. These lission fragments can be washed or dissolved from plates 51 by flowing water or a dissolving solution over them from solution pipe 54, the liquid then being conducted through product pipe 55 to tunnel 7 under control of valve 56. This highly radioactive material may be then deposited in the shielded cars 45 for removal to a concentration plant where the radioactive products can be separated for use as radiation sources as desired. The radioactive fission fragments enter the cooling stream due to iissions taking place at or close to the surface of the naked uranium bodies.

Due to the fact that a relatively large amount of such radioactive fission fragments may be present in the gas iiow from the gas outlet 26 to the precipitator 53 a heavy radiation shield 62 is placed around the gas line.

After the gases have passed through the precipitator 53, they enter a gas conduit 57 which turns downwardly to bend laterally again to join gas inlet 32, thereby closing the circulation path.

The gas is moved around the circulation path by one -or more blowers, such as fan 60 driven by motor 61. Fan 60 is preferably positioned at the beginning of the straight run of conduit 57 into gas inlet 32.

Some radioactive gaseous tission products can pass the precipitator 53 and later decay to radioactive solids. These solids may be deposited between the precipitator 53 and the fan 60, so this portion of the line is also provided with an extension of shield 62. Product pipe is also provided with a radiation shield 63. Deposited solid fission products can be removed from the gas line as desired by conveniently located solution sprays 64 and drains 65.

The system is enclosed in a welded gas-tight iron shell 63, all openings therethrough being suitably sealed. The system is maintained gas-tight throughout.

Figure 3 shows diagrammatically an open or, oncethrough system, where the gaseous heat transfer medium is air. In this case fan takes air from a filter 70 close to the ground level and forces this air through the reactor and the boiler. The exit air then passes through the precipitator 53 located in a stack 71 and is discharged from the top thereof. This stack is preferably made high, about 100 to 150 feet, so that gaseous fission products, and argon 4l formed in the air while in the reactor, will be greatly diluted before reaching the ground or contacting personnel in the vicinity. The fission fragments caught in the precipitator 53 are disposed of through the base of stack 71 through pipe 5S. The stack may be cleared of deposited fission products by spray nozzles 73 in the stack.

Referring now to the nuclear physics aspects of the reactor and the system, the closed circulatory system D will lirst be discussed.

As helium is a gas which does not absorb neutrons, it can advantageously be used for the gaseous transfer medium in the closed system and, due to the type of reactor used, the helium can be circulated at low pressure. As helium is also an inert gas, not combining with other materials, the reactor units can be made from materials that can not be used when air is the cooling medium. To charge the closed circulating system, it is exhausted or flushed and helium is supplied from helium container 75 through helium pipe 76 under control of helium valve 77. The overall pressure of the helium in the circulation system is preferably held just above atmosphere and the system welded gas tight. yIf leaks occur the slight positive pressure will force helium out, and thus prevent the entrance of air, with consequent oxidation of reactor materials.

In one form, the helium cooled reactor uses units approximating the shape of spheres of uranium carbide averaging about 2 inches in diameter as the fissionable element, and units approximating the shape of the same average diameter of graphite as the moderating element, with a volume ratio of graphite to uranium of about -l. As the relationship of moderator to uranium is not close to the optimum in this type of reactor due to the large number of voids the reactor will be relatively large. A cylinder about 36 feet high and 36 feet in diameter having a volume of 36,000 cubic feet filled with 2 inch units is used. The weight of the uranium component of the fuel is 100 tons, of graphite 860 tons. The reactor will, therefore, contain about 7,770,000 2-inch units. Void space with the 2 inch units is about 50 percent of the total volume, and the random adjacent mixing of the units provides an irregular body geometry where on the average every fourth or fifth unit in each direction is uranium. The units may be rough spheres one to three inches in diameter. The reactor as above described will be just above the critical size where the reproduction ratio is unity. When the control rod is removed from the reactor the neutron reproduction ratio in the reactor will be slightly above unity. With the rod removed, the neutron density in the reactor rises, heat is developed ase-aast and the helium blown through the reactor is heated. When the desired operating power is reached, the control rod is inserted to reduce the neutron reproduction ratio to unity, thereby holding the reactor at the power attained at the end of the neutron density rise.

In case of failure of the control rod to control the reaction, emergency measures can be taken, first, by dumping a portion of the units by use of grates 4, or Second by discharging water, a relatively good neutron absorber, onto the top of the reactor from jets 43. It has been found that there is no danger from the steam formedv in so discharging water onto units heated to 2000 C. Water is preferred for this use as no neutron absorbing residue is left in the reactor after use. It is, however, only used as a last resort emergency measure. Most of the heat is developed in the uranium carbide, but this heat is spread throughout the reactor by radiation, convection, and conduction. Due to the multiple,

irregular and interconnecting channels provided by the many voids, the cross section for the helium path can be considered as the cross section of the reactor, and at high temperatures the transfer of heat by radiation is important as it tends to keep the entire reactor at a substantially uniform temperature, thus minimizing overheating of the center of the reactor where the neutron density and consequently heat release is the greatest.

There will of course be a vertical temperature gradient due to the entrance of the cool gas at the bottom of the reactor.

, With the reactor operating to provide a maximum exit temperature of helium from the top of the reactor of 2000o C., the helium is circulated at the rate of 300,000

cubic feet per minute and passes through the boiler, lgiving up its heat to make steam. The helium is further cooled in the precipitator, and the water from the precipitator plates and from grates 4 can be used as preheated feed for the boiler if desired.

The maximum power is developed when 2000 C. helium is cooled to 100 C. in the boiler and circulatory system and then recirculated through the reactor; about 250,000 kilowatts will be carried away from the reactor.

This heat, when subjected to boiler losses and mechanical conversion efficiency losses, will provide about 80,000 kilowatts of electrical energy varying in accordance with the boiler efficiency and conversion to mechanical and electrical power.

` Due to the many voids, the pressure drop through the pebble bed reactor described is very low, about 3 pounds per square inch for 2 inch spheres, about doubleI that value for l inch spheres. This drop ismuch smaller than in gas cooled reactors using specific uranium body geometries, where the gas is passed only through narrow channels adjacent the surfaces of uranium rods or lumps. For this reason gas blowing costs in the pebble bed reactor are relatively low. Three 100,000 cubic feet blowers will handle the gas at an expenditure of only about 4500 kilowattsof energy. Y

When the pebble bed reactor is operated at the maximum temperature of 2000 C., uranium carbide Vis used because of its high melting point, about 2300" C. Further, due to the fact that boilers are not usually designed to handle a heat input as high as 250,000 kilowatts several boilers are preferably used, close together and manifolded to receive the hot helium gas.

However, it is not necessary to operate the pebble bed reactor at its maximum heat development of 2000u C., as it can be operated at lower temperatures and powers as desired. If temperatures below about 900 C. are used, uranium metal can be used for the units 35, the melting point of the metal being about 1100" C. At lowerpowers a corresponding saving is'made in blowing costs. Thus the power developed can be tted to various boiler designs.

It should also be understood that units 34 and 35, A

be of any shape, even highly irregular in surface, which whenY mixedv will provide the desired intercommunicating voids a'n'dV give th'e desired over all volume ratio of moderatoi' to uranium. These latter two factors are the essence of the present invention irrespective of how attained.

When the uranium carbideV and graphite moderator units are used,v the reflecting layer may be of graphite bricks. The use of the absorption space I4 outside of the graphite layer will next be considered. In any p'ower unit such as has ust been described the nuclear reaction fission's o'r burns the iissionable isotope U235 at the start of the-reaction, As the reaction continues the iissionable isotope 9423g is formed but not with unity conversion ratio and is also fissioned.V In conventional graphiteuranium reactors only about .80 atom of 94239 are formed for each atom of U235 destroyed by fission, a clear loss of iissionable isotope of 20 percent. As the total amount of uranium readily available in the world is presently throught to be limited (about 20,000 tons) the amount of U235, existing only as .7 percent of natural uranium is about 140 times less. In consequence it is desirable to increasethe flssionable isotope conversion factor as much as possible. Y

Irrespective of the presence of reflector 9 some neutrons are normally lost b'y escape beyond the reflector. While this loss varies with the size and composition of the reactor and recctor, 5 percent is a conservative ligure. The majority of these normally lost neutrons can be absorbed in a nonssio'nable isotope, such as thorium burned to 'tissionable isotopes formed, is raised by a few percent by the U233 production.

When air is used to cool the reactor and transfer the heat to the boiler, units of different materials are used in the reactor to avoid oxidation. In this case the moderator units are preferably made of beryllium oxide (BeO) compressed and sintered to density 3, and uranium oxide (U3O) compressed and sintered to density 6. While the uranium atom density in the oxide is less than in the metal or the carbide, the BeO moderator is more efficient than graphite. In consequence the two effects substantially balance leaving the reactor substantially the same size but with a slightly smaller ratio of moderator to uranium. When Be() is used as a moderator, BeO bricks are also used as the reiiector to avoid oxidation and BeO is also better than graphite as a reflector. This type of reactor can be operated up to a maximum of about 1500 C. outlet air temperature with a power output of about 100,000 kilowatts.

In reactors operating at high neutron `densities such as the reactor presently described, radioactive elements of exceedingly high capture cross section may be formed relatively quickly in the uranium as an intermediate element in the decay chains of the fission fragments and this formation can change vthe neutron reproduction ratio during operation if these elements remain in the reactor. One of the most important of these decay chains is believed to be the fission chain starting with Te (short) of which is about 30,000 10'24 cm.2. Upon absorption of a neutron, x'enonli5 shifts to xenon, an element of relatively small capture cross section.

Therrate of production of the Te is a function of the neutron density in which the uranium is immersed, and therefore dependent upon the power at which reactors of given type are operated. The radioactive Xenon135 is produced with a noticeable effect on the reaction a few hours after the reaction is started and the effect is, of course, greater as the neutron density is increased and maintained. The Xenonl35 effect in high power reactors can be summarized as follows when all the xenon remains in the reactor.

The reaction is started by withdrawing the control rod. The neutron density rises at a rate determined by the reproduction ratio and the effect of the delayed neutrons, until some predetermined neutron density is attained. The control rod is then placed in the unity reproduction ratio position and the reaction is stabilized at the power desired. During this time radioactive Te and iodine is formed, decaying to Xenon135. As more and more iodine decays, more and more Xenon135 is formed, this xenon135 absorbing sufficient neutrons to reduce the reproduction ratio below unity. This absorption also converts the Xenon to Xenon 136 which has no excessive capture cross section. The neutron density drops. If no compensation were made by the rod for this drop the density might drop until background conditions prevailed, and then the reaction might automatically start up as the Xenon135 decayed. Normally the neutron density drop is compensated for by removal of the control rod to a new position where the reproduction ratio is again above unity. A neutron density rise occurs, bringing the density back to its former level. Again, more xenon135 is formed and the process is repeated until an equilibrium condition is reached where the xenon135 formed is transmuted by neutron absorption and by decay into isotopes of lower capture cross section as fast as it is being formed. In the meantime, the control rod (or equivalent) has to be withdrawn by an amount thereby removing from the reactor, neutron absorbers at least equal in. effect to the absorption caused by the Xenon135.

In the reactors, as presently described, particularly when operated at high neutron densities, some of the telluriurn, iodine and Xenon135 will be diffused from the uranium into the cooling gas. In the closed gas circulation system, some of the circulating poisons will be returned to the reac-tor and some will decay to be picked up by the precipitator, or will be deposited on the gas line walls. With air, however, the diffused material will be completely removed from the system. Thus, in this respect, the air cooled reactor will require somewhat less of the reactive composition to maintain a reproduction ratio of unity at a given temperature (power) of operation than will the helium cooled reactor. As the amount of material of high capture cross section diffused from the uranium units will depend somewhat on the temperature at which the reactor is operated, critical size, during operation, that is, the size at which the reproduction ratio can be held at unity, and operating size, that is, a size only slightly above critical size at which the reproduction ratio can be slightly above unity, are readily obtained by changing the actual size of the reactor. The reactor described can be enlarged as desired by loading more units on top, or reduced by discharging units from the bottom. Thus, the reactor can be changed in size as desired to compensate for changes in reproduction ratio due to changes in operating conditions of any sort. Such changes in size can also be made to accommodate long term changes in reproduction ratio, such as a size due to accumulation of element 94 which is somewhat more eiiicient than U235 as a fissionable material, or such as a reduction due to neutron absorption in accumulation of stable residual fission fragments in the uranium.

Enrichment of the uranium in ssionable isotope such as for example, by raising the U235 content or adding 94239 vor H233 will permit reduction in reactor size in accordance with the amount of enrichment made. Too great a reduction in size however is not preferred as it reduces the cross section of the gas path through the reactor and thereby increases the pressure drop at high power operation. However, the present invention is clearly applicable to enriched reactors of any size, and the advantages of the pebble bed reactor herein disclosed will still apply. Further, the present invention contemplates a pebble bed -reactor in which the units 34 and 35 each contain moderator and fissionable material.

summarizing, it will be seen from the above description that the pebble bed reactor is ideally suited to supply hot gases resulting from the heat of a nuclear reaction, at temperatures sufficiently high to be useful and efficient to produce power. These advantages can be summarized as follows:

(l) Simplification of circulating system and reduction of cost of cooling due to low pressure drop.

(2) Uniformity of temperature in reactor through thermal radiation at high temperatures.

(3) Increased thermodynamic efficiency for power purposes because of high heat transfer efficiencies.

(4) Simplicity of structure as exact geometrical spacing of the uranium bodies is not used.

(5) No cooling pipes or foreign material (other than control rod and sheath) are required inside reactor, thus reducing parasitic neutron losses.

(6) Simultaneous production of power and fissionable isotopes suitable for re-use.

(7) High isotope conversion factor-both 94 and U233 are produced during operation.

(8) Last and perhaps most important, the pebble type reactor supplies hot gas within the temperature ranges and at gas pressures presently obtained by burning combustible fuel such as gas, coal and oil, thereby permitting the utilization of the heat of the nuclear reaction in standard steam plants of existing designs, in binary systems, such as mercurysteam systems, or in gas turbines.

While the pebble bed reactor construction described herein has been cooled by a gas, this type of reactor can be modified for use with a liquid coolant, as set forth in my copending application Serial No. 621,844, tiled Cctober ll, i945.

While the theory of the nuclear chain fission mechanism in uranium set forth herein is based on the bestpresently known experimental evidence, it is not desired to be bound thereby, as additional experimental data later discovered may modify the theory disclosed.

'What is claimed is:

l. in a neutronic reactor system, a core capable of sustaining a controlled chain reaction comprising a right cylindrical chamber at least 36 feet high and 36 feet in diameter, containing roughly spherical units of uranium carbide and of graphite, the diameter of said uranium carbide units and of said graphite units being between about one and three inches, said uranium carbide and said graphite units being randomly intermingled so that on the average at least every fifth unit in any direction is a fuel unit, the weight of the uranium component of the uranium carbide units being at least about l0() tons and the weight of the graphite being at least about 860 tons, a neutron reflector radially surrounding said core, a helium coolant, control means, means for passing helium through said core, and means for removing the heat imparted to the helium in the reactor core from the helium.

2. in a neutronic reactor system, a reactor core capable of sustaining a controlled chain reaction comprising a cylindrical chamber at least 36 feet high and 36 feet in diameter, containing roughly spherical units of uranium carbide and of graphite, the diameter of the uranium carbide units and of the graphite units being about two inches, said uranium units and graphite units being randomly interminglcd so that on the average at least every fifth unit in any direction is a uranium unit, a void space between the units of approximately 50 percent of Athe total core volume, the weight of the uranium conl- References Cited in the le of this patent UNITED STATES PATENTS 2,500,223 Wel'lsetal ...7---- Mar. 14, 1950 2,708,656 Fermi et a1. a May 17, 1955 10 FOREIGN PATENTS Australia May 2, 1940 'France Oct. 28, 1940 Switzerland Oct. 2, 1944 Great Britian Jan. 3, 1951 OTHER REFERENCES Goodman: The Science and Eng. of Nuclear Power, vol. 1, page 275, Addison-Wesley (1947).

Kelly et al.: Phy. Rev. 73, 11359 (1948).

Nucleonics, lune 1953, pp. 23, 50, 51, 52, 53. 

1. IN A NEUTRONIC REACTOR SYSTEM, A CORE CAPABLE OF SUSTAINING A CONTROLLED CHAIN REACTION COMPRISING A RIGHT CYLINDRICAL CHAMBER AT LEAST 36 FEET HIGH AND 36 FEET IN DIAMETER, CONTAINING ROUGHLY SPHERICAL UNITS OF URANIUM CARBIDE AND OF GRAPHITE, THE DIAMETER OF SAID URANIUM CARBIDE UNITS AND OF SAID GRAPHITE UNITS BEING BETWEEN ABOUT ONE AND THREE INCHES, SAID URANIUM CARBIDE AND SAID GRAPHITE UNITS BEING RANDOMLY INTERMINGLED SO THAT ON THE AVERAGE AT LEAST EVERY FIFTHUNIT IN ANY DIRECTION IS A FUEL UNIT, THE WEIGHT OF THE URANIUM COMPONENT OF THE URANIUM CARBIDE UNITS BEING AT LEAST ABOUT 100 TONS AND THE WEIGHT OF THE GRAPHITE BEING AT LEAST ABOUT 860 TONS, A NEUTRON REFLECTED RADIALLY SURROUNDING SAID CORE, A HELIUM COOLANT, CONTROL MEANS, MEANS FOR PASSING 